Very high-frequency electron tube



Feb. 5, 1Q52 c. w, HANSELL VERY HIGH-FREQUENCY ELECTRON TUBE Filed Jan. 18, 1947 zdf i JNVENTOR. BY

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Patented Feb. 5, 1952 VERY HIGH-FREQUENCY ELECTRON TUBE Clarence 1W..Hansell, Port Jefferson, N..Y., as,- signor to Radio Corporation of America, a corporation of Delaware Application January 18, 1947, Serial No. 722,833

'1o claims.

This invention relates to electron tubes especially adapted for use at very high frequencies, and particularly to an electron -tube capable of operating on a band of frequencies such as would be required in a wide band amplifier used as a repeater on coaxial lines, wave guides, and radio relay circuits.

In the construction of a growing wave amplier tube, sometimes called a travelling wave tube, a coil or helix type of radio frequency transmission line is used which may be as long as 40 or 50 wavelengths peripherally at the operating frequency. This type of tube functions by virtue of the mutual interaction and growth of waves on the coiled or helical line and Waves in the space charge of an electron stream made up of electrons moving inside of. and lengthwise of the coiled line at velocities somewhat greater than the velocities of the waves along the turns of the coil. For a general description of this type of tube reference is made to U. S. Patent 2,300,052 granted October 27, 1942, to Nils E. Lindenblad, and to the article in Electronics for November, 1946 entitled Wide band microamplifier wave tube, pages 90 to 92 and to the article in the November, 1946 issue of Wireless World, "Ihe travelling wave valve, pages 369 to 372.

It is known to couple the input and output circuits for the tube to the ends of the coiled or helical line. Such known arrangements may involve a connection to the input end of the coiled line from an input wave guide for bringing small wave energy to the tube, and a connection to the output end of the helix from an output wave guide taking away large wave energy from the tube.

Experience with coiled lines indicates that it may be diicult to transfer power to and from a coiled transmission line in this manner. Generally it is found that, if a certain value of alternating current potential is applied at one end of a coiled line, the resulting potential appearing at the other end of the line is very small. The line in this respect behaves like a low pass electrical lter and, if the intended operating frequency is above the cut-off frequency region,

high filter type attenuation results.

The reason for this filtering action is that the` effective characteristic impedance of a coiled transmission line, which will be relatively high but uniform near the middle portion of its length is subject to rapidly decreasing values as the ends are approached. This decreasing characteristic impedance as the ends are approached comes 2 about because the turns of the coiled line near the end are coupled mutually to fewer neighboring turns than are turns near the center of the line and the effective characteristic impedance is dependent to a large degree upon the mutual couplings between turns.

I have found that if the connections to the coiled line or helix are moved progressively inward from the ends and not made to the extreme ends of the coiled line, coupling or connection points or regions will be found at which the system as a whole acts substantially as though it were a transmission line of uniform impedance, free from low pass lter type attenuation, and this overcomes the foregoing difliculty.

In practice, depending on the pitch and diameter of the coil (helix) and the frequency range of operation, the connections from the input and output circuits may be moved in from the end of the coil for distances corresponding to as much as a half wavelength of Wire, measured around the helix. As an example, if the operating frequency is 1,000 ,megacycles it may be found advantageous to make connections to the coil at points such that the end turns of the helix outside the connecting points at each end of the coil comprise about '7 or 8 centimeters of Wire. By trial, some broad optimum position of connecting points may be found.

`In cases where the wavelength of waves along the coil, measured axially, becomes of the order of magnitude of the diameter of the coil, the cut-off characteristic of the end portion of the line may be so great that what is required is simply to make a connection far enough from the end to get on the edge of the region of high attenuation. I have found cases where good results were obtained by connecting to the coil at a point in from the end about 1A wavelength of wire, assuming the velocity of light, measured from the end and following the wire peripherally around the helix.

In cases where the wavelength of waves along the coil, measured axially, is large compared with the coil diameter, good results may be obtained by grounding, or bypassing with capacity, the extreme ends of the coil to a surrounding metal shield and then tapping or connecting the input and/ or output circuits to the coil at points of the order of 1/4 wavelength from the ends, as measured around the turns of the coil.

Broadly, the present invention comprises a means to improve growing wave electron tubes by making input and output connections not to the extreme ends of the coiled growing Wave transmission line but to points which are located inward from the ends sufficiently to overcome high attenuation of the end portions of the line, caused by rapidly changing characteristic impedance near the ends.

The invention is described in more detail in the following description in conjunction with a drawing, wherein;

Fig. 1 illustrates an electron tube embodying the invention;

Figs. 2 and 2a are sections of the device of Fig. 1 looking at the electron tube from the input and output ends of the helix, respectively.

Referring to the drawing,V there. is` shown. a wide band amplifier tube comprisingat one end. an electron gun having a cathode K and an electron beam forming tubular element A', a coiled line or helix H surrounding and closely coupled to the electron stream passing axially through its interior, and a collector electrode C all within an evacuatedglass envelope E. Surrounding the glass envelopeA isa metallic shield M. A suitable source of D. C. power supply, P furnishes polarizing potentials to the various electrodes. It should be noted that the positiveterminal of the power supply P is connectedto Iground and also to the metallic shieldM and to the collector electrodeC. Cathode heating` power of low frequency is suppliedto Atransformer T whose secondary winding is connected to the heater of the cathode K. An input ,wave guide IG. supplies the incoming signalto be amplified to the helix or coiled. line H. An output Wave guide OG serves t0 deliverthe energyv extracted from the helix to a suitable load, not shown.

In accordance with. the invention, the input Wave guide and theoutput waveguide are coupled to the helix H by means of connections or coupling rods B and ,.D, respectively, which are connected to the helix at points some distance removed from the ends. These points of connection to the helix Hare so chosen as to overcome the high attenuation of the waves near the ends of the coil caused by the rapidly changing characteristic impedance of the coiled line H near its ends.

A magnetic eld coil F suitable energized from asource l0 serves to provide an axial magnetic iield for preventing an undesired spread of the electron beam as it travels from the electron gun K to the collector electrode C. The electrons or charged particles are shown as a stream S.

From an inspection of Figs. 2 and 2a which show respectively features looking toward the ends of. the helix or coiled line, it will be seen that the input and output wave guides each include an adjustable coupling iris R and an adinstable-tuning piston V. The iris and tuning piston enable a proper impedance match between the wave guide and the helix at the location of coupling.

The helix H may be as much as 40 or 50 wavelengths long at a mean operating frequency. This mean operating frequency may be anywhere in the range of 800 megacycles to 5,000 megacycles and higher, depending upon the design of thev amplifier. The system is so chosen that the velocity of the electrons along the stream S is somewhat greater than the velocity of the travelling waves induced in the coil H. In accordabove, the pitch of the coil or helix may betvaried to hold a correct relation between velocities of traverse the coil due to energy transferred to the waves on the coil.

'Ihe ratio of the inside diameter of the metallic envelope M and the diameter of the helix H is so chosen as to obtain the necessary coupling between the helix and the electron stream. The diameter of the helix is also such as to permit the desired internal andl axial eld components to react with the axially moving electron stream. Obviously, the metallic shield M may be omitted entirely, but use of the shield is preferred in any case to reduce radiation and coupling from the coil to the surrounding space.

Others working with growing wave tube have found that a material for the helix which is of relatively large resistance is better than one of relatively low resistance, because large attenuation is an aid to prevention of uncontrolled oscillation -due to reection of wave power from the output to the input end of the coil. My coupling arrangement, by providing a better imv pedance match over a wider frequency band reance with the Lindenblad patent referred to i waves on the coil and in the electron stream,.;

even though the electrons slow downas .theyj the electron collector electrode.

duces the amount of loss or wave attenuation required and so improves the power efficiency and power output of the device.

Generally speaking, the points of connection from the input and output coupling rods B and D to the helix H may be about 1/4 of a wavelength from the ends. as measured around the turns of the helix, although in some cases it depends on the pitch and the diameter of the helix, and the range of frequencies. These points of connection may vary and may correspond almost to as much ,as a 1/2 wavelength of wire from the end of the helix measured around the helix. The optimum point of connection may be found by trial and this method is recommended in each specicy 'amplifier generally followsV established principles in that the electron stream is bunched to some degree by the incoming waves to be amplifled, and this bunching of the electrons increases as portions of the electron stream move toward The' Wave induced on the helixby the moving electron stream is a traveling wave and this traveling wave, as it grows, also causes a growth in electron bunching in the electron stream.

What is claimed is:

l. An electron tube comprising means for producing a stream of charged particles, a helical conductor located adjacent the path of said stream and adapted to be in energy coupling rela- "tion to the stream, and coupling means adapted to carry radio frequency currents coupled to a point on said helical conductor removed from one end by a distance measured around the helix which is at least of the order of one-quarter wavelength at the operating frequency.

2. An electron tube comprising an envelope having an electron gun and an electronA collector spaced therefrom, means for causing electrons to flow in a stream from said gun to said collector, a coil surrounding the space between said gun and collector in which said stream is adapted to pass, said coil being adjacent the path of said stream and adapted to be in energy coupling rela- -Ztion to said stream, and radio frequency input @around the coil.

3. An electron tube comprising an envelope having an electron gun and an electron collector spaced therefrom, means for causing electrons to flow in a stream from said gun to said collector, a coil surrounding the space between said gun and collector in which said stream is adapted to pass, said coil being adjacent'l the path of said stream and adapted to be in energy coupling relation to said stream, and a radio frequency output coupling means coupled to a point on said coil removed from the end nearest said collector by a distance of the order of one-quarter of a Wavelength at the operating frequency measured around the coil.

4. An electron tube comprising an envelope having an electron gun and an electron collector spaced therefrom, means for causing electrons to ow in a stream from said gun to said collector, a coil surrounding the space between said gun and collector` in which said stream is adapted to pass, said coil being adjacent the path of said stream and adapted to be in energy coupling relation to said stream, and a radio frequency input coupling means coupled to a point on said coil vremoved from the end nearest said electron gun by a distance of the order of one-quarter of a wavelength at the operating frequency measured around the coil, and a radio frequency output coupling means coupled to a point on said coil removed from the end nearest said collector by a distance of the order of one-quarter of a wavelength at the operating frequency measured around the coil.

5. In a wideband Very high frequency amplifier adapted to operate at frequencies of the order of 1,000 megacycles and higher, an electron tube having means for producing an electron stream over a path, a helix adjacent said path and in energy coupling relation to any stream of charged particles which may pass over said path, and means for supplying very high frequency waves to be amplified directly to a point on and removed from one end of said helix by a distance measured around the helix in the range of substantially one-quarter to one-half wavelength at the operating frequency.

6. In a wideband very high frequency amplier adapted to operate at frequencies of the order of 1,000 megacycles and higher, an electron tube having means for producing an electron stream over a. path, a helix adjacent said path and in energy coupling relation to any stream of charged particles which may pass over said path. and means for extracting very high frequency waves from said helix, said means being connected to a point on and removed from one end of said helix by a distance measured around the helix in the range of substantially one-quarter to one-half wavelength at the operating frequency.

7. An electron tube comprising means for producing a stream of charged particles, a helical conductor located adjacent the path of said stream and adapted to be in energy coupling relation thereto, and a radio frequency carrying coupling means coupled to said helical conductor at a point thereon removed from one end thereof 1ly us; substantial distance measured around the 8. An electron tube comprising an envelope having an electron gun and an electron collector spaced therefrom, means for causing electrons to flow in a stream from said gun to said collector, a coil surrounding the space between said gun and collector in which said stream is adapted to pass, said coil being adjacent the path of said stream and adapted to be in energy coupling relation to said stream, and a waveguide having a coupling probe therein coupled to a point on said coil removed from the end nearest said electron gun by a distance of the order of one-quarter of a wavelength at the operating frequency measured aound the coil. v 9. An electron tube comprising an envelopel having an electron gun and an electron collector spaced therefrom, means for causing electrons to flow in a stream from said ygun to said collector, a coil surrounding the space between said gun and collector in which said stream is adapted to pass, said coil being acacent the path of said stream and adapted to be in energy coupling relation to said stream, and a waveguide having a coupling probe therein coupled to a point on said coil removed from the end nearest said collector by a distance of the order of one-quarter of a wavelength at the operating frequency measured around the coil.

10. An electron tube comprising means for producing a stream of charged particles, a helical conductor located adjacent the path of said stream and adapted to be in energy coupling relation thereto, and radio frequency input and output coupling means coupled to said helical conductor adjacent the opposite ends thereof. each of said coupling means being coupled to said conductor at a point thereon removed from the respective end thereof by a substantial distance measured around the helix.

CLARENCE W. HANSELL.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,064,469 Haeff Dec. 15. 1936 2,108,900 Peterson Feb. 22, 1938 2,128,234 Dallenbach Aug. 30, 1938 2,300,052 Lindenblad Oct.'27. 1942 2,368,031 Llewellyn Jan. 23, 1945 OTHER REFERENCES Article by J. R. Pierce, Bell Lab.V Record, Dec. 1946,pgs. 439-442, incl. Y

Electronics for Nov. 1946, pgs. 90, 91,192.

Publication. pgs. 57 and 103, Electronic Industries, for Dec. 1946, vol. 5, No. 12. 

